This invention relates to differential pressure indicators commonly found in hydraulic systems.
In any hydraulic system wherein fluid such as oil or the like is passed through a filter, it is customary to provide an indicator for indicating when the filter has become loaded with contaminants and requires cleaning or replacement. Since the pressure drop across a filter element increases in proportion to the accumulation of contaminants thereon, a suitable indication can be obtained by an indicator that is actuated when the differential pressure across the filter element reaches a predetermined value. Since the viscosity of fluids normally increases as temperature decreases, pressure drops across the filter element at unusually low temperatures can be higher than the predetermined value, causing false indications even though the filter element is clean. Various temperature control devices have been utilized for preventing false indications at low temperatures.
Pall, in U.S. Pat. No. 2,942,572 issued June 28, 1960, discloses a magnetic differential pressure indicator. In this indicator, a first magnetic element is arranged to attract a second magnetic element as long as two magnetic elements are separated by less than a predetermined distance. The first magnetic element is movable reciprocally with a piston responsive to changes in pressure, and is normally biased toward the second magnetic element by a first spring with predetermined load. The second magnetic element is also movable reciprocally with a piston, and while retained in its un-actuated position by magnetic attraction, is normally biased in a direction away from the first magnetic element by a second spring capable of overcoming the force of magnetic attraction whenever the first and second magnetic elements are separated by the predetermined distance. The second spring propels the second magnetic element to an indicating position whenever the predetermined distance is exceeded. In order to prevent actuation at low temperatures, a bimetallic element is positioned to prevent motion of the second magnetic element. The bimetallic element is comprised of two arcuate inner and outer strip portions of different metals or metal alloys having different coefficients of thermal expansions and joined together by a weld and arranged to bend inwardly with decreasing temperature. At temperatures below 32.degree. F., for example, the bimetallic element contracts inwardly over a flange on second magnetic element, thus preventing a false indication.
Since the second magnetic element is retained in the un-actuated position by the magnetic attraction between the first and second magnetic elements, as the amount of magnetic attraction decreases (which occurs as the square of the distance), a position develops such that the magnetic attraction is only slightly greater than the force of the second spring continuously urging the second magnetic element away from the first magnetic element. Under such circumstances the application of a slight force, such as by vibration, G-forces or the like, to the indicator of U.S. Pat. No. 2,942,572 would cause the separation of the magnets and thus the indication of the existence of a clogged condition even though such does not in fact exist.
Silverwater, in U.S. Pat. No. 3,785,332, issued Jan. 15, 1974 discloses a magnetic differential pressure indicator in which a loose nonmagnetic detent ball is provided to prevent resetting of the indicating signal after actuation, thus assuring that cleaning or replacing of the clogged filter in fact has been performed if the indicator is in its reset state. The detent ball falls down from its orignal position into a recess after actuation in order to prevent a second magnetic element from being pushed close to a first magnetic element. Thus, the second biasing spring can always overcome the magnetic attraction and keeps the second magnetic element in the indicating position. Since the differential pressure indicators may be used in aircraft applications, they must necessarily be small in size and light in weight. The detent ball of U.S. Pat. No. 3,785,332 therefore is very small and light. Once the fluid such as oil accumulates around the detent ball, it will stick to the surrounding wall and thus be unable to fall down in order to perform its function. Another disadvantage of the indicator of U.S. Pat. No. 3,785,332 is that since the indicator has to be inverted to allow the detent ball to fall back to its original position, that indicator can only be installed on removable parts, such as a filter casing. This greatly limits its application.
To overcome the above deficiencies, attempts have been made to utilize devices for positively locking the first magnetic element in a position whereat the magnetic attraction is less than the biasing force of the second spring. In this case, false indication at low temperature cannot be prevented simply by preventing motion of second magnetic element at low temperatures. This is because the first magnetic element will be moved toward and locked in position due to high pressure drops that will occur at low temperatures. As the temperature exceeds a certain level, the second spring will overcome the magnetic attraction and cause a false indication. Since the bimetallic elements are inherently fragile, they will be damaged if arranged to restrain motion of the first magnetic element. To overcome these difficulties, another Silverwater reference, U.S. Pat. No. 4,172,971, discloses a magnetic differential pressure indicator. In this indicator, the first magnetic element is arranged movably with a tubular sleeve concentric to and slidable within an enclosing hollow piston. A bimetallic element is disposed inside the tubular sleeve. When temperature is above the certain or selected value, the tip of the bimetallic element is turned to extend through a passage in the tubular sleeve and projected into a groove on the piston, so that the tubular sleeve and hollow piston move together so that the first magnet is responsive to the differential pressure. At low temperatures, the tip of the bimetallic element is out of the groove in the piston; therefore the sleeve is not linked to the piston, so that the first magnet does not respond to the differential pressure. This prevents false indication at low temperatures. A detent spring is provided as the positive locking element. It engages the sleeve by entering a groove on the sleeve when the sleeve moves with the piston to the predetermined distance. One disadvantage of this device is that due to dimensional tolerance of individual pieces, the detent spring may have engaged the sleeve before actuation occurs, or may not engage the sleeve when actuation already occurs. This either results in an inoperative indicator or loss of the non-resettability feature until more contaminants are built up. Another disadvantage of this indicator is that while it is not desirable to have too much clamping force imposed on the sleeve by the detent spring, which clamping force the bimetallic element has to overcome, it is required that the detent spring be able to contract to engage the groove on the sleeve and withstand the first spring biasing force. This requires precision calibration. A further deficiency of the device of U.S. Pat. No. 4,172,971 is that when one pushes on the sleeve to reset the indicator internally, damage of the bimetallic element is likely to occur, since the tip of the bimetallic element is still in the groove on the piston if the temperature of the fluid is above the preselected value.
Conventional filter devices are normally incorporated with a by-pass mechanism that is responsive to a predetermined pressure drop across the filter element. Above this predetermined pressure drop, the by-pass mechanism is opened and unfiltered, contaminated fluid is circulated through the system. The by-pass mechanism is normally set to open when the filter element is further clogged beyond the level required to actuate the clogged condition indicator. Thus, the by-pass mechanism serves as a safety device when cleaning or replacement of the filter element is neglected or when it is impossible to service the filter, for example, under in-flight conditions. Up to now, aircraft specifications have required the use of two separate indicators for a filter, to indicate clogging of the filter element and that opening of the by-pass mechanism is about to occur or has occurred, respectively. Such duplication of indicators results in increased costs and cumbersome configurations. In addition to the aforesaid disadvantage, all of the prior art devices fail to indicate when no filter element is installed in the fluid system. When a system is functioning with no filter element to filter out the contaminants, even for a relatively short time, the resulting damage to the system may be very great. If the system is in operation on an airplane during flight, this may even result in loss of power to the aircraft and danger to human lives. In present filter systems, a safeguard against operation without a filter is provided by a mechanism that prevents installation of the filter housing without a filter element in place. This mechanism results in increased cost and additional weight.